1. Field of the Invention
The present invention relates to an outer tube for a front fork using a fiber reinforced plastics (FRP) pipe.
2. Description of the Related Art
In an inverted type front fork of a two-wheeled motor vehicle, an inner tube on the side of an axle is slidably inserted into an outer tube on the side of a vehicle body, and since a diameter of the outer tube is greater than that of the inner tube, rigidity of the outer tube is greater than that of the inner tube.
According to an outer tube described in Japanese Patent Application Laid-open No. 2000-18303 (patent document 1), bending rigidity of a lower portion of a vehicle body mounting portion in the longitudinal direction of the vehicle body is increased so that the outer tube is less prone to be deformed, and bending rigidity in the lateral direction of the vehicle body is reduced so that the outer tube is prone to be deformed. With this structure, tightness feeling with respect to a load in the longitudinal direction of the vehicle body applied to a front fork at the time of braking is secured, and shock absorbing performance with respect to a load in a widthwise direction of the vehicle applied to the front fork at the time of cornering is enhanced.
According to an outer tube described in Japanese Patent Application Laid-open No. 2006-347386 (patent document 2), bending rigidity of a lower portion of a vehicle body mounting portion at a position at 45° from the longitudinal direction of a vehicle body is increased so that tightness feeling with respect to a load in the longitudinal direction of the vehicle body applied to a front fork at the time of braking is secured, and shock absorbing performance with respect to a load in a widthwise direction of the vehicle applied to the front fork at the time of cornering is enhanced.
In the outer tube for the front fork described in the patent document 1, an outer peripheral surface is cut so that the bending rigidity in the lateral direction of the vehicle body is reduced. The addition of this feature is complicated.
In the outer tube for the front fork described in the patent document 2, another member is adhered to an outer peripheral surface of the outer tube, thereby increasing the bending rigidity at a position at 45° from the longitudinal direction of the vehicle body. The addition of this feature is also complicated.
When the outer tube is comprises an FRP pipe, the number of laminated resin-impregnated fiber sheets (prepreg sheets) of the FRP material is adjusted in a circumferential direction of the pipe, and a thickness of the FRP pipe is varied in the circumferential direction of the pipe, the bending rigidity of the outer tube in the longitudinal extent of the vehicle body and the lateral extent of the vehicle body. However, it is complicated to adjust the number of laminated prepreg sheets of the FRP material in the circumferential direction of the pipe, which increases cost.